Paying For Child That Lives With Me

My boyfriend has twin boys he has paid child support but one of his sons live with us and has for over a year. We have been fighting custody for him since Aug 09. They are 17 yrs old. He has back child support and when we get custody can we fight for the year of child support she owes him. She keeps fighting us we have ask for reviews for his child support nothing yet please advise us what we can do thank you